Clothes dryer designs

[Stephen Tashiro]

Are their any clothes dryers that pull air directly from outside the dryer to go over the heating element and into the drum?

The dryers I've taken apart pull air from inside the dryer cabinet to go over the heater and into the drum. I can see that this design might be more efficient since the air inside the cabinet is pre-heated by what's going on inside. But the inside of the cabinet gets a lot of lint in it and the air inside is probably more humid that the outside air.

[Bill Howatt]

Most of our dryer use is in the winter and -20F or colder outside air to heat up isn't exactly what we are looking for. I agree outside air is dryer in the winter but in an AC house it certainly isn't in the summer. Of course, things in NM aren't the same.

[Stephen Tashiro]

Most of our dryer use is in the winter and -20F or colder outside air to heat up isn't exactly what we are looking for..

I'm thinking of "outside" air as being the air in the room where the dryer sits - as opposed to the air inside the dryer chassis. (Of course, if it sits in an unheated area, I suppose -20 F is possible.)

[Mark Wedel]

The air in the dryer chassis is coming from somewhere - most likely from the room itself. I could imagine there is a slight increase in efficiency by using the chassis air, as that is probably warmer than the room air (which will replace it)
I've not looked, but wonder what electric heat pump dryers do - in theory, those could be more closed loop - one side of the heat pump warms the air, passes it over the clothes, this hot and humid air then passes over the cold side of the heatpump, which dehumidifies the air and recovers some of the heat before going back to the hot side. Though in that case, you would need a drain for water so condensed, which might be more an issue for some dryer install locations.

[Stephen Tashiro]

The air in the dryer chassis is coming from somewhere - most likely from the room itself..

Given how much lint builds up inside dryers, I'd say a lot of the air in the chassis is leaking out from the drum. I guess that counts as recycling some air.

[Bill George]

The air for the dryer comes from room air and then goes thru a Lint filter and should be exhausted to the outside the building air. Otherwise the moisture would never be removed from the wet clothes.

[Roger Feeley]

I think we are missing the point here. Stephen is right. I have never seen a clothes dryer with a defined air inlet. The air comes from the room and through the cabinet to a blower. I don’t doubt that this leads to lint buildup inside the cabinet.

so the question is whether there’s a dryer out there that has an air inlet duct.

my answer might not be exactly what you want Stephen. I have a ductless dryer. They are very popular in the EU and are catching on here to in apartment and condos. In my dryer, air flow is a closed circuit. Air flows through the drum and picks up moisture. Then it goes through a dehumidifier condenser. At some point, it blows excess heat into the room but it’s dry air. The water is collected and is pumped out to a drain. I have to remove the condenser plates a couple of times a year and rinse off the lint. No big thing.

so, as far as I know, my dryer would meet Stephens test.

[Bill Howatt]

I'm not clear on the lint build-up mechanism but I have seen it in old dryers so I'm certainly not denying it.
The lint comes from the clothes being dried - not the room. So why is it worse that the air comes from the room rather than an outside duct. The lint is in the airstream after it goes through the dryer, not before. Is the issue, that the "hot waste air" is not being ducted to the filter properly?

[Stephen Tashiro]

The air for the dryer comes from room air and then goes thru a Lint filter and should be exhausted to the outside the building air. .

I agree that it should do that theoretically. However, since lint comes out of the drum and gets into the chassis, it's clear that some air is also coming out of the drum - at least in the dryers I have opened up. A dryer designed so the blower sucks in room air through an inlet in the chassis and blows it directly into the drum would use room air exclusively. But that is not how the dryers I have seen are designed.

[Stephen Tashiro]

Is the issue, that the "hot waste air" is not being ducted to the filter properly?

If you are talking about the issue of lint build up inside dryers, yes, it is because not all the hot waste air is ducted through the filter. There are many felt seals in dryers, including a felt seal around either rim of the drum. I think these are a leaky type of seal.

Before I took apart some dryers, my guess about their operation was that there would be a direct path from the outside air over the heating element and into the drum. This is not the case. In the typical dryer, a blower sucks air out of the drum and blows it out the back of the dryer. The inlet to the heating element is not directly connected to room air. It takes in air from inside the chassis near the front fo the dryer. There is a vent to admit room air at the back of the dryer and the chassis itself is not air tight. Lint from the chassis can get sucked over the heating element since the inlet to the heating element has no filter. It's common to find burned bits of lint around the heating element in a used dryer.

In spite of that disadvantage, I assume there are reasons why this design is used.

[Tom M King]

Not exactly to the point of this thread, but something else to think about with dryers. When I was building new houses, I had to design them anyway since a lake house needs two fronts-one on the road side and one on the water side. One thing I always did was put the dryer backing up against an outside wall so it direct vented through the wall, so that no flex hose was used or needed. I expect we all know how lossy flex hose is, and that they always accumulate fine lint from dryers. The wall vent was removable and in a panel that it could be located in a different spot in the future if needed. You could install the wall vent after the dryer was in place.

[Lee Schierer]

My pet peeve with dryers is why the interior cabinet duct work is so convoluted and small that it is impossible to clean your the lint that gets past the lint filter.

[Stephen Tashiro]

One thing I always did was put the dryer backing up against an outside wall so it direct vented through the wall, so that no flex hose was used or needed.

I'm curious how you arranged that. Did you have a standard type dryer vent with a door flap to keep bugs out? How do I arrange the connection so the dryer can be pulled away from the wall when the need arises. One thing a flexible hose does is give enough slack to pull the dryer out enough to work behind it.

[Tom M King]

It goes directly into a standard wall vent with flap. Once the vent is in the right place, the dryer can be taken out and put right back. Just a friction fit but the dryer stays in place fine. Feel in the cap from the outside to make sure it’s being slid in correctly. We’re probably on our fourth dryer here, and I’ve never even needed to move the wall cap.

[Tom M King]

I have one I can take a picture of tomorrow in the rental house. It goes out the side to a direct vent through the wall. Since it’s out the side I can see it with the camera.